1. Field of the Invention
The present invention relates to an image reader and an image forming apparatus, and more particularly to an apparatus provided with plural pairs of rollers arranged at the upstream side and the downstream side of an image reading position.
2. Description of the Related Art
Conventionally, in a image reader or an image forming apparatus having an image reader, a original conveying apparatus, which is one example of a sheet conveying apparatus for conveying an original that is a sheet, is provided, and when an image of the original is read, the original is conveyed onto an original base plate glass by the original conveying apparatus. The image of the original is read by image reading unit located below the original base plate glass. There has also been known, as the conventional image reader, the one including two image reading portions each provided with image reading unit in order to enhance productivity, wherein the front and back surfaces of the original are read during one conveyance.
In the conventional original conveying apparatus provided to the image reader described above, the original placed onto a original tray is fed by a pickup roller, separated one by one at a separation portion, and then, conveyed to the image reading portion. A pair of first conveying rollers is provided at the upstream side of the image reading portion, while a pair of second conveying rollers is provided at the downstream side of the image reading portion. When the image of the original is read, the original is conveyed as nipped between the pairs of first and second conveying rollers.
If the sheet conveying speeds of the pairs of first and second conveying rollers are the same when the original passes through the image reading portion, the original is conveyed without being bent. However, the sheet conveying speeds of the pairs of first and second conveying rollers are not the same in actuality since there is a variation in precision of parts.
When the diameter of the roller of the pair of first conveying rollers is greater than the diameter of the roller of the pair of second conveying rollers, for example, the original conveying speed of the pair of first conveying rollers is larger than that of the pair of second conveying rollers during the same period, even if the pairs of first and second conveying rollers convey the original with the same revolution. In this case, the original is bent on the reading portion. When the pairs of first and second conveying rollers are further rotated, the deflection is increased, and hence, the state of the original on the reading portion is not stabilized to deteriorate the reading quality.
In view of this, in the conventional image reader, the diameter of the roller of the pair of second conveying rollers is set greater than the diameter of the roller of the pair of first conveying rollers by the amount greater than the variation in the precision of parts. Thus, the original is pulled by the pair of second conveying rollers, with the result that the original is conveyed as being stable on the reading portion without being bent.
However, even when the roller diameter of the pair of second conveying rollers is set to be greater than the diameter of the roller of the pair of first conveying rollers, the conveying force of the pairs of first and second conveying rollers changes due to the variation in the roller pressure, when the roller pressures (nipping pressures) of the pairs of first and second conveying rollers are set to be equal to each other. In this case, the original cannot stably be conveyed. Therefore, the conveying force of the pair of second conveying rollers, which pulls the original, is set to be smaller than the conveying force of the pair of first conveying rollers. This structure prevents the pairs of first and second conveying rollers from excessively pulling at each other in order to avoid the excessive conveying load and the application of the load to the original.
However, even when the situation described above is set, there arises a phenomenon in which the reading quality is severely deteriorated, depending upon the property of the conveyed original, when the trailing end of the original passes through the pair of first conveying rollers. It is supposed that the original is a thick paper having a great grammage, for example. In this case, the moment the trailing end of the original completely passes through the pair of first conveying rollers having a great conveying force, by which the conveying force of the pair of first conveying rollers is not exerted to the original, the conveying force is temporarily reduced, resulting in that the reading quality is deteriorated.
It is supposed that the original is a thin paper having a small thickness. In this case, the moment the trailing end of the original completely passes through the pair of first conveying rollers, and then, is conveyed only by the pair of second conveying rollers, the original is temporarily pulled by the pair of second conveying rollers having a great conveying speed, so that the reading quality is deteriorated.
Therefore, in order to prevent the deterioration in the quality caused by the rapid change in the conveying condition that is generated when the original completely passes through the pair of first conveying rollers, there has conventionally been proposed a measure in which the conveying force of the pair of first conveying rollers is substantially continuously lowered before the trailing end of the original completely passes through the pair of first conveying rollers. According to this measure, the rapid reduction of the conveying force can be avoided with respect to the thick paper. With respect to the thin paper, the pair of conveying rollers that mainly conveys the thin paper is gradually changed from the pair of first conveying rollers to the pair of second conveying rollers, so that the rapid speed change can be avoided.
The pair of conveying rollers includes a drive rotating body that rotates by power transmitted from driving unit and a driven rotating body that rotates as being in press contact with the drive rotating body. The pair of conveying rollers is configured such that plural drive rotating bodies and the driven rotating bodies are equally spaced or arranged at predetermined positions in the main scanning direction. The conventional pair of first conveying rollers and the pair of second conveying rollers have plural pairs of rotating bodies, e.g., five pairs of rotating bodies, including the drive rotating body and driven rotating body.
In a state (conveying state) where all of the pairs of rotating bodies constituting the pair of conveying rollers are in press contact with a certain press-contact force, the pair of conveying rollers has the conveying force capable of conveying even the thickest paper assumed by the image reader. On the other hand, in order to lower the conveying force of the pair of conveying rollers, the press-contact force of all of the plural pairs of rotating bodies is reduced more than the press-contact force in the conveying state (separating state). There has been proposed an apparatus in which the press-contact force of all pairs of rotating bodies is set to be zero in the separating state, i.e., all the pairs of rotating bodies are completely separated.
On the other hand, the reading quality is also deteriorated when the leading end of the original reaches the press-contact portion between the drive rotating body and the driven rotating body of the pair of second conveying rollers, in the case where the original is a thick paper. This is considered to be caused as described later. Specifically, when the leading end of the original enters the press-contact portion between the drive rotating body and the driven rotating body, the original does not smoothly enter since the thickness of the paper is great. Therefore, the conveying resistance is produced instantaneously, whereby the conveying speed of the original is reduced. As a countermeasure of this situation, the pair of second conveying rollers is completely separated at the timing when the leading end of the original reaches the pair of second conveying rollers.
In order to prevent the deterioration in the reading quality, the pairs of first and second conveying rollers are appropriately changed between the conveying state and the separating state (see Japanese Patent Application Laid-Open Nos. 2001-2277 and 2003-222957).
When the image of the original is read, for example, the pair of first conveying rollers is firstly brought into the conveying state, and the pair of second conveying rollers is brought into the separating state. Thus, the original is conveyed as nipped between the pair of first conveying rollers, and the leading end of the original reaches the image reading portion with this state. The reading operation is started at the timing when the leading end of the original reaches the image reading portion. The original is further conveyed, and then, the leading end of the original reaches the pair of second conveying rollers that is in the separating state. By virtue of this structure, the deterioration in the reading quality can be prevented.
When the leading end of the original reaches the pair of second conveying rollers, the pair of second conveying rollers is changed from the separating state to the conveying state. After that, when the trailing end of the original approaches to some degree to the pair of first conveying rollers, the pair of first conveying rollers starts to change from the conveying state to the separating state, and the pair of first conveying rollers is brought into the separating state immediately before the trailing end of the original completely passes through the pair of first conveying rollers. When the trailing end of the original completely passes through the image reading portion, the reading operation is completed.
There may be the case in which a second image reading portion, which reads the surface reverse to the surface read by the reading operation, is provided at the downstream side of the pair of second conveying rollers, wherein the above-mentioned reading operation and the reading operation by the second reading portion are done during one conveyance of the original. In this case, the pair of second conveying rollers is brought into the separating state immediately before the trailing end of the original reaches the pair of second conveying rollers.
In the conventional sheet conveying apparatus, the deterioration in the reading quality can be prevented by changing the states of the pairs of first and second conveying rollers to the conveying state or to the separating state depending upon the type of the original. In order to achieve this operation, detecting unit, which determines the type of the original as needed, is required.
However, providing the detecting unit is difficult from the viewpoint of cost. Even if the detecting unit is provided, it is difficult to determine a thickness or size of the original one by one conveyed before the start of the reading operation as well as keep high productivity, since the original processing speed in the image reading is increased in recent years. Therefore, in the conventional sheet conveying apparatus, the changeover of the state of the pairs of first and second conveying rollers is performed in the same manner for all of various types of originals assumed by the image reader in its specification.
Further, it is difficult to prevent the instantaneous rapid change in the conveying state when the trailing end of the original completely passes through the pair of first conveying rollers, even if the press-contact force of the pairs of rotating bodies, which are provided at five positions, is lowered. This is because, even if the press-contact force is lowered, all of five pairs of rotating bodies are in press contact. Specifically, since all of five pairs of rotating bodies are in press contact, the portion where the original and the pairs of rotating bodies are brought into contact with each other is great. Therefore, greater friction force is generated between the pairs of rotating bodies and the original at the contact portion. As described above, when all of the pairs of rotating bodies are in press contact, the nipping force for nipping the original remains even if the press-contact force is lowered.
Specifically, even if the press-contact force of the pair of first conveying rollers is lowered, the original is substantially nipped between the pair of first conveying rollers due to the friction force between the pair of first conveying rollers and the original. Therefore, the rapid change in the conveying state at the time when the trailing end of the original completely passes through the pair of first conveying rollers cannot be prevented. This tendency is increased particularly when a thick original having a small size (A5, STMT, B6) and having a great grammage is conveyed.
The operation of the pair of first conveying rollers at the upstream side of the position where the image is read has been described above. For the pairs of rotating bodies at the downstream side from the position where the image is read, when the press-contact force for all the pairs of rotating bodies is lowered, the same problem arises as in the pair of first conveying rollers.
When the press-contact force of the pairs of rotating bodies arranged at five positions is set to be zero, the rapid change of the conveying state at the time when the trailing end of the original completely passes through the pair of first conveying rollers is prevented, but there arises a problem that the original is likely to be skew-fed the moment the press-contact force is set to be zero. This is because unit for preventing the vibration at the trailing end of the original is eliminated. Accordingly, a method for preventing the instantaneous rapid change in the conveying state at the time when the trailing end of the original completely passes through the pair of first conveying rollers is needed upon conveying a thick original having a small size.
Upon conveying a small-sized original, there my be the case in which the timing when the leading end of the original reaches the pair of second conveying rollers and the timing when the trailing end of the original reaches the pair of first conveying rollers are overlapped, so that both the pair of first conveying rollers and the pair of second conveying rollers are brought into the separating state. When both the pair of first conveying rollers and the pair of second conveying rollers are brought into the separating state, the original cannot be conveyed. When the conveying force of the pair of first conveying rollers and the conveying force of the pair of second conveying rollers are both lowered, slippage occurs between the conveying rotating body and the original in the case of a thick paper, resulting in that paper jam might be generated.
Upon conveying a small-sized thin original, when the nipping force of the pair of first conveying rollers and the nipping force of the pair of second conveying rollers are both lowered, harmful effects are likely to be brought about. Specifically, since the thin original is susceptible to the effect caused by the conveyance, compared to the thick original, even if the press-contact force of the conveying rotating body is small, the original is liable to be skew-fed or wrinkles are liable to be generated in the conveying direction. Therefore, the reading quality might be significantly deteriorated, or the original itself might be damaged.
In view of the circumstance described above, the present invention provides an image reader and an image forming apparatus that can stably convey an original (sheet) and can prevent the deterioration of the reading quality.